The present invention relates to an improved eddy current probe and a method for using the same and, more particularly, to a flexible eddy current probe that may be utilized to inspect blade roots or disk slots of different types of blades and disks having different surface characteristics.
An eddy current (EC) is an electrical current induced in a conductor by reaction with a magnetic field. Eddy Currents are circular in nature with paths oriented perpendicular to the direction of the applied magnetic field. Eddy current probes, as well as ultrasonic probes, have been utilized to non-destructively inspect surfaces of gas turbines, aircraft engines, and so on, and have especially been utilized to inspect the rotating disks and blade roots thereof.
In general, varying magnetic fields during EC testing are generated by an alternating current flowing through a coil positioned immediately adjacent to the conductor. The magnetic field can vary in magnitude and distribution in relation to the following attributes of the specimen or part being inspected: (1) electrical conductivity; (2) magnetic permeability; (3) geometry; and (4) homogeneity. To yield useful and accurate information from an EC inspection, one must isolate and examine those areas or portions of the specimen on the part undergoing inspection. In order to generate the eddy current, the conductor must remain at the same distance and angle from the surface of the specimen. This distance must be maintained for every subsequent specimen due to component geometry dimensions. The eddy current will decrease as the distance from the coil or surface of the part increases. In practice, eddy current strength drops off so rapidly that the currents are negligible and become undetectable by conventional instrumentation a relatively short distance from the coil. In all instances, the physical size of the conductor housing must allow the probe or coil to be placed consistently in the same position relative to the area being inspected. This effect or response due to geometry changes and coil-to-part spacing is called lift-off In addition to the particular geometry of the part being inspected in relation to the physical characteristics of the EC probe, changes in lift-off also result from surface roughness, slight contour, probe wobble, probe bounce and inconsistencies in the thickness of nonmetallic coating, such as paint, primer, and anodic coating. Due to impedance changes caused by lift-off variations, defects, for example, large cracks or other flaws, in the surface under inspection may not be readily detected by the EC inspection.
In order to overcome the above-mentioned problem, it is necessary to maintain the EC probe at the same angle and distance to the part as the probe passes across its surface. This, however, is virtually impossible to accomplish with a single hand-held EC probe and, thus, the industry has developed and fabricated geometry-specific conductor housings designed to conform to the specific shapes of the parts being inspected. Generally, an EC coil within a particular EC probe is positioned within a block of non-conductive material that is permanently shaped to fit a unique geometry. By utilizing numerous geometry-specific probes to carry out inspections of complex machine or structures, such as highly stressed aircraft frames, turbine blades and disk components, structural flaws and defects are detected with a high degree of accuracy.
Over the past ten years a vast amount of work has been done to expand the coverage of EC probes in order to reduce inspection time. For example, the aircraft industry has developed fixtures that allow the placement of several EC coils within a single fixture design (multi-coil arrays), in accordance with standard practice, to match a part's specific geometry. This example is depicted in a turbine disk. Due to the nature of these highly stressed parts, it is important that each disk slot be inspected along the pressure faces of both sides. Through the use of computer controls, the inspection technician can be assured that all surfaces are scanned with a very high degree of reliability. However, and although this technique is very sensitive and effective, it requires multiple probes, fixtures, extensive personnel training and exceedingly costly inspection devices. Using such inspection equipment and techniques, inspection time still is exceedingly large and it is known to take as long as twelve hours to inspect a turbine disk. Such lengthy inspections result in increased downtime and increased costs.
Thus, current EC probe inspections suffer from the following: (1) a unique probe is required for each different part undergoing inspection; (2) probe failure requires multiple probes for each part; (3) multiple calibration standards are required; (4) a separate probe calibration is required for each type of part inspected; (5) extensive development efforts and time are required to make new probes; and (6) substantial expense is incurred since EC probes are expensive.
It is therefore an object of this invention to provide an improved eddy current probe that overcomes the above-mentioned disadvantages of prior EC probes.
It is another object of this invention to provide an eddy current probe that can be used to interrogate parts with different surface structures and geometries.
It is a further object of this invention to provide an eddy current probe that reduces inspection time.
It is still another object of this invention to provide an eddy current probe that allows for the inspection of the entire surface of the part under inspection in only a single pass of the probe.
Various other objects, advantages and features of the present invention will become readily apparent to those of ordinary skill in the art, and the novel features will be particularly pointed out in the appended claims.